Molecular sieves are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework-type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, 5th edition, Elsevier, London, England (2001).
Among said zeolitic materials, Chabazite is a well studied example, wherein it is the classical representative of the class of zeolitic materials having a CHA framework structure. Besides aluminosilicates such as Chabazite, the class of zeolitic materials having a CHA framework structure comprises a large number of compounds further comprising phosphorous in the framework structure are known which are accordingly referred to as silicoaluminophosphates (SAPO). In addition to said compounds, further molecular sieves of the CHA structure type are known which contain aluminum and phosphorous in their framework, yet contain little or no silica, and are accordingly referred to as aluminophosphates (APO). Zeolitic materials belonging to the class of molecular sieves having the CHA-type framework structure are employed in a variety of applications, and in particular serve as heterogeneous catalysts in a wide range of reactions such as in methanol to olefin catalysis and selective catalytic reduction of nitrogen oxides NOx to name some two of the most important applications. Zeolitic materials of the CHA framework type are characterized by three-dimensional 8-membered-ring (8MR) pore/channel systems containing double-six-rings (D6R) and cages.
U.S. Pat. No. 7,067,108 B2 discloses zeolites of Chabazite framework type. These zeolites are prepared by employing a specific seeding material, namely a crystalline material having a framework type other than Chabazite framework type, such as AEI type, LEV type, or OFF type, in addition to N,N,N-trimethyl-1-adamantylammonium hydroxide used as the structure directing agent.
U.S. Pat. No. 6,974,889 B1 on the other hand discloses a process for the manufacture of a crystalline molecular sieve, such as zeolites of structure type CHA or LEV, containing phosphorus in its framework, wherein tetraethyammonium hydroxide is used as the templating agent, and wherein a colloidal crystalline molecular sieve is used as seed material. In particular, said document teaches the use of seed crystals having the structure type LEV, OFF, or CHA, wherein said seed crystals should be as small as possible for controlling the particle size of the product as well as for accelerating its formation. Specifically, the synthesis of SAPO-34 is disclosed in said document using colloidal solutions of Chabazite crystals.
Although some progress has been achieved regarding the costs of the organotemplate used in the synthesis of CHA-type zeolites, as well as with respect to the duration of the synthetic process, the major drawback remains with respect to the necessary use of a structure directing agent which must be subsequently removed. In this event, the organotemplates are contained in the pore structure of the resulting zeolite, such that it may first be effectively employed in an application only after removal thereof. Furthermore, the organotemplate may usually only be removed by a calcination process or the like, such that a recycling of the organotemplate is not possible. Another disadvantage concerns the decomposition of the organic template material during hydrothermal synthesis, which not only makes it necessary to employ reaction vessels displaying a high pressure resistance, but also limits the possibility of recycling materials used in synthesis due to the presence of waste products from organic decomposition. As a result of these constraints, the known procedures for the production of CHA-type zeolite materials are highly cost-intensive, making these zeolites unattractive for a variety of applications.
Furthermore, the necessary removal of the organotemplate by calcination at higher temperatures, normally at 450 to 930° C. or even higher, is not only disadvantageous due to the destroyal of costly organic template, but also results in excess energy consumption and produces harmful gases and other unwanted waste products. In addition to this, the harsh thermal treatment ultimately limits the types of architectures which may be provided according to the known production methods. In particular, although ion exchange methods for extracting the organotemplate from the zeolitic material have been developed as an environmentally friendly alternative to calcination for removing the organic template, only part of the organic templates may successfully be recycled, the remainder interacting too strongly with the zeolite framework for complete removal. Accordingly, the synthesis of CHA-type zeolite materials which are devoid of an organotemplate remains effectively limited to those materials capable of withstanding the harsh conditions necessary for the complete removal of the organotemplates necessarily used in the synthesis thereof. As a result of this, the harsh thermal treatment ultimately limits the production to thermally stable CHA-type zeolite materials, in particular to those which display a high SiO2:Al2O3 molar ratio.
In Hasegawa et al., Journal of Membrane Science 2010, 347, 193-196, a process for the preparation of a Chabazite-type zeolite layer on a porous α-Al2O3 tube is disclosed wherein said synthesis is achieved by the use of seeding crystals having the CHA framework structure in combination with the use of strontium as the structure directing agent in the seeded synthesis, wherein the seed crystals have been obtained from inter-zeolitic transformation of Y-type zeolite into CHA-type zeolite. Li et al. in Microporous and Mesoporous Materials 2011, 143, 270-276, on the other hand, also reports the synthesis of Chabazite-type zeolite layers on porous α-Al2O3 substrates using CHA-seeding materials obtained from inter-zeolitic transformation of Y-type zeolite into CHA-type zeolite, wherein, however, as opposed to Hasegawa et al., potassium is employed as the structure directing agent in the synthetic process. Said processes are, however, highly limited by the fact that the CHA-type zeolitic material used as the seeding agent is specifically obtained from inter-zeolitic transformation, as a result of which the CHA-type zeolitic materials which may be obtained from such processes are equally very limited. In particular, the Si to Al ratios available for said seeding materials obtained from inter-zeolitic transformation are highly limited such that only materials having very low Si to Al ratios may be used. Thus, the seeding material employed in Hasegawa et al. displays an SiO2:Al2O3 molar ratio of only 4.8 for obtaining a product having an SiO2:Al2O3 molar ratio of 6.4. Li et al., on the other hand, only allows for the production of zeolitic materials having the CHA-type framework structure with an SiO2:Al2O3 molar ratio of 5.72.
Consequently, there remains a considerable need for a process for the organotemplate-free production of a zeolitic material having a CHA-type framework structure which may provide a large number of different zeolitic materials having a wide range of physical and chemical properties which is in particular reflected by the SiO2:Al2O3 molar ratios present in said materials. Furthermore, there is a particular need for the provision of an organotemplate-free synthetic process which is truly template-free and not bound to the specific use of other ions as structure directing agents which is again highly limiting with respect to the range of different zeolitic materials which may be produced and the specific respective chemical and physical properties which may be obtained.
Furthermore, apart from the considerable restrictions of the processes known from the prior art, there also remains a considerable need for a cost efficient process which affords a microcrystalline product, the processes respectively known from Hasegawa and Li et al. involving the use of bulky support materials and excessive amounts of seeding material compared to the actual amount of zeolite obtained. Furthermore, the products in Hasegawa and Li et al. may only be obtained in the form of membrane layers supported on bulky composite support materials respectively consisting of a specific zeolite seeding material provided on a solid support.
Accordingly, there also exists a need for an organotemplate-free synthetic process which may provide a microcrystalline zeolitic material having the CHA-type framework structure.